PRL family phosphatases (PRL-1, PRL-2 and PRL-3) are highly attractive targets for developing inhibitors as novel anti-cancer therapeutics because their over-expression has oncogenic effects and plays a potentially pathogenic role in human malignancies. Our recent studies provided the first evidence that pentamidine (PE), an anti-protozoan drug with an unknown mechanism of action, is an effective inhibitor of PRLs and has anti-cancer effects. The goal of this study is to test our hypothesis that PE has growth inhibitory effects against tumor cells via inactivating PRLs through direct binding to the PTPases and has therapeutic potential against malignancies associated with PRL over-expression. Aim 1. To evaluate the roles of PRL PTPases in mediating PE anti-cancer effects, we will 1) determine the capacities of Peinsensitive mutants of PRL-1, PRL-2 and PRL-3 individually and in combinations to confer resistance to Peinduced growth inhibition in WM9 human melanoma cell line to define the contributions of each PRL in PE effects against the cancer cells and in cancer cell growth; 2) determine PE activity to reverse transformed phenotypes induced by over-expression of PRLs or PTPalpha oncogene in NIH3T3 cells to further elucidate PE anti-cancer mechanism; 3) determine PE effects on lung metastasis of A549 transfectants of PRLs in nude mice to provide prove of concept evidence regarding PE anti-metastasis activity and the roles of individual PRLs in tumor metastasis; 4) identify human malignancies over-expressing PRLs by RNA in situ hybridization to assess the significance of individual PRLs as anti-cancer targets. Aim 2. To elucidate inhibition mechanism of PE against PRLs, we will 1) identify key chemical groups in PE essential for PRL inhibition and PTPase specificity by evaluating 10 PE-derivatives in which PE chemical groups have been selectively modified; 2) characterize PE / PRL complex formation and PE effects on PRL-1/substrate interaction by mass spectrometry and biochemical analysis; 3) analyze solution structures of PRLs and their complexes with PE by NMR. Results of the study will provide proof of principle evidence to establish PE as a PRL-targeted agent with anti-cancer potential, reveal the inhibition mechanism of PE against PRLs at molecular levels, provide insights for rational design and development of more specific and less toxic novel PRL inhibitors and identify human malignancies potentially responsive to PRL-targeted therapy.